1. Field of the Invention
The invention concerns wet-lay papermaking and improving the performance of urea-formaldehyde wet strength resins in making wet strengthened paper.
2. Description of the Prior Art
The principal method of incorporating urea-formaldehyde wet strength resins into a sheet of paper is by adding the wet strength resin to a pulp slurry at some point in the wet end (white water system and pulp slurry system) of the papermaking process. For example, wet strength resins and other chemicals have been added to the beater chest, quick mix chest, machine chest, storage chests, suction side of the fan pump (head box pump), and other convenient points for addition of chemicals to the papermaking system prior to sheet formation. During sheet formation the pulp slurry exits the head box and the pulp is deposited upon a foraminous surface while water passes through the foraminous surface. The white water pasing through the sheet is usually collected in paper machine vessels such as the white water chest, couch pit, or riffler where the white is accumulated and recycled in the papermaking process.
Chemicals added to the pulp slurry for imparting strength to the paper product are classified as either dry strength additives or wet strength additives. Dry strength additives such as carboxymethylcellulose break down when exposed to water and lose their strength imparting capabilities. Wet strength additives resist the action of water to impart strength to the paper when wet.
Most wet strength resins added to the papermaking slurry have an affinity for the cellulosic fibers in the papermaking slurry. The resins' affinity for papermaking fibers is obtained by ionic charge. Such resins are referred to in the industry as substantive resins. Cationic resins naturally have the appropriate affinity for cellulosic fibers. Resins can be made cationic by incorporating some cationic components into the resin during the manufacture of the resin. Urea-formaldehyde resns are made substantive to cellulosic fibers by incorporating some cationic charge into the urea-formaldehyde polymer. This is usually accomplished by including a cationic monomer such as an amine along with the urea and formaldehyde during the manufacture of the urea-formaldehyde resin. Such cationically modified urea-formaldehyde resins are referred to as substantive urea-formaldehyde resins and are disclosed in U.S. Pat. Nos. 3,275,605, issued Sept. 27, 1966; 2,554,475, issued May 22, 1951 and 2,683,134 issued July 6, 1954.
Substantive urea-formaldehyde resins have been used to impart wet strength to paper by incorporating the urea-formaldehyde resin into the pulp slurry prior to sheet formation. Drying of the sheet after formation partially cures the urea-formaldehyde resin attached to the fibers comprising the sheet. However paper made with urea-formaldehyde resin requires natural aging of several weeks before the paper achieves its full wet strength potential. In fact, paper coming off the papermaking machine has only a small fraction (e.g. about 30% or less) of the wet strength potential obtainable upon aging.
Water soluble salts of carboxymethylcellulose have been used in papermaking as a dry strength additive. Because of the water solubility of carboxymethylcellulose, paper having enhanced dry strength due to the presence of carboxymethylcellulose readily disintegrates in the presence of water (lacks wet strength). Accordingly carboxymethycellulose is referred to as a dry strength additive usually used in the manufacture of corrugated board and other paper products such as kraft paper which require high strength but not necessarily wet strength.
U.S. Pat. No. 3,630,831 is an example of the many disclosures teaching the use of carboxymethylcellulose as a binding agent for non-woven materials. U.S. Pat. No. 2,766,137 discloses the use of an ammonium salt of carboxymethylcellulose for treating fibrous products to impart some wet strength. However, the salt is decomposed into the free acid which is further decomposed to the "refractory" state.
U.S. Pat. No. 2,297,698 is one example of the use of water soluble cellulosic compounds such as hydroxyalkylcellulose and the alkali metal salts thereof in combination with other ingredients such as urea-formaldehyde resin and polyhydric alcohol for treating bibulous paper.
The above patents do not teach the synergistic affect of salts of carboxymethylcellulose upon the cure rate of substantive urea-formaldehyde resin when added to the pulp slurry which is particularly surprising since the carboxymethylcellulose salt by itself does not impart any wet strength nor does the combination significantly affect the final wet strength of the paper.
One function of a wet strength resin is to impart sufficient strength to the paper while still wet on the machine in order to minimize paper breaks during high speed papermaking. A second function of a wet strength resin is to impart the desired degree of wet strength to the final product which depends upon the desired end use of the paper product. A disadvantage of urea-formaldehyde resin is that the quantity needed to perform the first function (strengthen the sheet to minimize breaks on the papermaking machine at high speeds) is often much larger than the quantity of resin needed to give the ultimate wet strength desired for the paper product after aging. This excess quantity of urea-formaldehyde resin needed to impart sufficient strength on the papermaking machine is wasted in the ultimate product and a method of increasing the cure rate of the urea-formaldehyde resin to increase the on machine wet strength would be highly desirable and eliminate this waste of urea-formaldehyde resin.
U.S. Pat. Application Ser. No. 419,022, filed Nov. 26, 1973, now U.S. Pat.No. 3,953,638 inventor Clifford B. Kemp, available as a priority document for French patent disclosure 2,252,212, published on July 25, 1975 discloses the ability of carboxylmethylcellulose to improve the wet strength of paper in combination with a wet strength resin such as Kymene S-2064. Kymene S-2064 is an alkaline-curing wet strength resin believed to be a polyaminepolyamide-epichlorohydrin resin (cationic) available from Hercules Incorporated, Wilmington, Delaware. In contrast, the present invention does not improve the wet strength of paper produced utilizing urea formaldehyde resin but improves the cure rate (off machine strength) of the paper without significantly affecting the ultimate wet strength. The relevant disclosure in U.S. Pat. Application Ser. No. 419,022 is on page 30. However, the publication date of July 25, 1975 is after the conception and reduction to practice of the present invention.